The invention generally relates to apparatus and methods for measuring blood pressure in animals, and more particularly continuous nonblood pressure measurement apparatus and methods.
In the case of a hospitalized patient, it is frequently desired to be able to measure the blood pressure on a continuous basis. A very reliable technique for continuously measuring blood pressure is to insert a saline filled catheter through the patient""s vascular system to the point at which it is desired to perform the measurements. The catheter is connected to a pressure sensor, which measures the pressure in the vessel. An alternative method uses a catheter with a pressure sensor at the tip that directly senses the blood pressure. However these techniques involve making an incision through the patient""skin and inserting the catheter into a blood vessel. As a consequence, this invasive procedure entails some risk of complications for the patient.
An indirect, non-invasive process for continuously measuring blood pressure is based on the pulse transit time (PTT) which is the time required for a blood pressure pulse from the heart beat to propagate between two points in the vascular system. One apparatus for this technique includes an electrocardiograph which senses electrical signals in the heart to provide an indication when a blood pulse enters the aorta. A pulse oximeter is placed on an index finger of the patient to detect when the blood pressure pulse reaches that location. The pulse transit time between the heart and the index finger is measured and calibrated to the existing blood pressure that is measured by another means, such as by the automated oscillometric method. Thereafter changes in the pulse transit time correspond to changes in the blood pressure. Generally the faster the transit time the higher the blood pressure. Thus changes in the pulse transit time can be equated to changes in the blood pressure.
However, the electrocardiograph (ECG) senses electrical signals in the heart, which do not indicate the point in time when the blood pressure pulse actually leaves the heart upon the mechanical opening of the aortic valve. A time interval of varying length, known as the cardiac preperiod (PEP), exists between peaks of the QRS wave of the electrocardiogram signal and the aortic valve opening. The inability of prior pulse transit time based monitors to account for the cardiac pre-injection period resulted in an inaccurate measurement of the pulse transit time and thus blood pressure.
In addition changes in the compliance of the blood vessels also affects the pulse transit time. Chronic changes in arterial compliance occur due to aging, arteriosclerosis, and hypertension. Arterial compliance can also change acutely due to neural, humoral, myogenic or other influences. Previous monitoring systems have been unable to separate changes due to compliance from changes due to blood pressure. As a consequence, some degree of inaccuracy exists in calculating blood pressure from the variation of the pulse transit time.
Thus there is a desire to provide a more accurate continuous, non-invasive blood pressure measurement technique.
An apparatus for continuously and non-invasively monitoring blood pressure includes a device that measures impedance at a first location in the animal. The resultant impedance measurements are employed to detect when a blood pressure pulse occurs at the first location. A sensor is provided to detect when a blood pressure pulse occurs at a second location in the animal. A timer, that is connected to the device and to the sensor, measures the interval of time, which elapses between when blood pressure pulses occur at the first and second locations. A processor employs conventional pulse transit time analysis techniques to derive the blood pressure of the animal from the interval of time measured by the timer.
In one embodiment of the present invention an impedance cardiograph is employed to measure the thoracic impedance of the animal. Analysis of the thoracic impedance measurement determines when the aortic valve of the animal""s heart opens, thereby emanating a blood pressure pulse from that first location. A pulse oximeter can be used as the sensor to detect when a blood pressure pulse occurs at a second location in the animal.
In another embodiment, an impedance plethysmograph measures the impedance at two locations on a limb of the animal to detect when a blood pressure pulse occurs at those locations.
Other aspects of the present invention involve use of an electrocardiograph, which along with the impedance cardiograph enables calculation of the cardiac stroke volume. The cardiac stroke volume then is employed to compensate the blood pressure derivation for effects due to blood vessel compliance. In addition, a non-continuous blood pressure monitor can be periodically activated to produce a reference blood pressure measurement that is used to calibrate the pulse transit time analysis.